In the past several years, stents have emerged as a prime therapy for atherosclerosis, because they counteract the effects of intimal hyperplasia from balloon injury. Unfortunately, in-stent restenosis can occur as a result of stent injury to the vessel wall. In response, drug eluting stents have been developed, which have a polymeric coating that releases a drug at a rate and for a duration that is effective to counteract the effects of in-stent restenosis. Examples of such devices include drug eluting coronary stents, which are commercially available from Boston Scientific Corp. (TAXUS), Johnson & Johnson (CYPHER), and others. The polymeric coating on the stent is in contact with the delivery system (e.g., balloon) on its inner diameter and in contact with the vessel wall on its outer diameter. It is therefore advantageous to optimize the properties of the polymeric coating so as to control the release of drug, to have optimum biocompatibility against the vessel wall, and to be compatible with the surface of the balloon.
Poly(styrene-isobutylene-styrene) triblock copolymers (SIBS), described, for example, in U.S. Pat. No. 6,545,097 to Pinchuk et al., are thermoplastic elastomers in which the poly(isobutylene) mid-block is elastomeric and the styrene end blocks form physical crosslinks. This polymer is used in drug-releasing polymeric coatings for coronary stents. The polymeric coating has good integrity and, being elastomeric, is able to expand as the stent is expanded. SIBS also has excellent biocompatibility, particularly within the vasculature.
SIBS is made by a living cationic polymerization process, which must be conducted at low temperatures and under stringent conditions. If possible, it would be advantageous to manufacture polymers that are analogous to SIBS polymers using less stringent processes, such as such as free radical polymerization.
Furthermore, there are a limited number of monomers that can be polymerized by living cationic polymerization, restricting one's ability to vary the chemical composition of polymers made by this process. Because the chemical and physical characteristics of polymers are strongly influenced by the monomers that are used to form them, it would be advantageous to employ other processes besides living cationic polymerization. For example, the number and chemical variety of the monomers that can be polymerized by free radical processes are considerably broader that those that can be polymerized by cationic methods. The ability to use free radical processes would therefore allow one to tailor the properties of the polymer with greater flexibility as compared to cationic polymerization methods.